Genetic studies of structural anomalies in humans help to better understand the underlying basis for normal and abnormal embryological development. We have studied the most common brain anomaly in humans, holoprosencephaly (HPE), a structural defect of the developing forebrain and midface. HPE is a genetically heterogeneous disorder associated with various chromosomal anomalies. Recently, mutations in the human Sonic Hedgehog (SHH) gene were shown to cause familial forms of HPE. Furthermore, anomalies in the cholesterol biosynthesis were found in a genetic syndrome associated with HPE. Thus, other yet unidentified HPE causing genes are postulated to be part of the SHH signaling pathway or are involved in the cholesterol biosynthesis. Lanosterol synthase (LS), is the key enzyme involved in the first step of the cholesterol synthesis. The LS gene has been mapped to human chromosome 21q22.3, a region known to be deleted in some HPE patients. Mutational analysis of the LS gene in HPE patients is now in progress to determine whether LS is another HPE candidate gene.